There are a number of types of apparatus where a plurality of chambers or systems need to be evacuated down to different levels of vacuum. For example, in well known types of mass spectrometer, the analyser/detector has to be operated at a relatively high vacuum, for example 10−5 mbar, whereas a transfer or optics chamber, through which ions drawn and guided from an ion source are conveyed towards the detector, is operated at a lower vacuum, for example 10−3 mbar. The mass spectrometer may comprise one or more further chambers upstream from the analyser chamber, which are operated at progressively higher pressures to enable ions generated in an atmospheric source to be captured and eventually guided towards the detector.
Whilst these chambers may be evacuated using separate vacuum pumps, each backed by a separate, or common, backing pump, it is becoming increasingly common to evacuate two or more adjacent chambers using a single, “split flow” pump having a plurality of inlets each for receiving fluid from respective chamber, and a plurality of pumping stages for differentially evacuating the chambers. Utilising such a pump offers advantages in size, cost, and component rationalisation.
For example, EP-A 0 919 726 describes a split flow pump comprising a plurality of vacuum stages and having a first pump inlet through which gas can enter the pump and pass through all of the stages, and a second inlet through which gas can enter the pump at an inter-stage location and pass only through subsequent stages of the pump. The pump stages can be configured to meet the pressure requirements of the chambers attached to the first and the second inlets respectively.
International patent application no PCT/GB2004/004046 filed by The BOC Group plc; the contents of which are incorporated herein by reference, describes a split flow pump in which a pump inlet for receiving gas from a high pressure chamber is located between stages of a multi-stage Holweck molecular drag mechanism. FIG. 1 is a cross-sectional view of part of a split flow pump 10 similar to the pump described in that application. The Holweck mechanism comprises two co-axial cylindrical rotor elements 12a, 12b of different diameters, preferably formed from a carbon fibre material, mounted on a disc 14 located on the drive shaft 16. A stator for the Holweck mechanism comprises two cylindrical stator elements 18a, 18b co-axial with the rotor elements 12a, 12b to define, in this example, three pumping stages comprising three annular pumping chambers 20, 22, 24 located between the rotor elements 12a, 12b and the stator elements 18a, 18b. The surfaces of the stator elements 18a, 18b which face a rotor element are formed with helical channels 26 in a manner known per se and as shown in FIG. 2.
The pump 10 has a first inlet (not shown) through which gas (indicated by arrows 36 in FIG. 1) enters the pump 10 and passes through all of the chambers 20, 22, 24 of the Holweck mechanism before being exhaust from the pump 10 through pump outlet 28 located in the base 30 of the pump 10. A second, interstage inlet 32 is located between the stages of Holweck mechanism so that gas (indicated by arrow 38 in FIG. 1) entering the pump through the interstage inlet 32 passes into an annular plenum 34 located between the pumping chambers 20 and 22, from which the gas 38 passes through fewer chambers of the Holweck mechanism (chambers 22 and 24 in this example) than the gas 36 before being exhaust from the pump 10 through pump outlet 28. This can provide for differential pumping of a system attached to the inlets.
With an even distribution of gas flow/pressure in a Holweck stage, each individual channel 26 of the stage is subject to the same boundary conditions (flow and pressure) and so provides the same level of performance. This is the most efficient operating condition of the Holweck stage. For instance, in the example shown in FIG. 1 gas passing through the outermost annular chamber 20 will be flowing evenly though all of the helical channels 26 of the annular chamber as it leaves the annular chamber 20. In the absence of any interstage flow 38, the gas will simply continue to flow in this manner round to the next downstream chamber 22 meaning an evenly distributed flow/pressure and good stage performance.
Now consider the other extreme case of gas distribution from the interstage inlet 32 in the absence of any gas 36 from the first inlet. The interstage gas load enters the pump 10 at a single point on the circumference of the interstage plenum 34. This gas then attempts to distribute itself around the plenum 34 prior to being pumped through the downstream annular chamber 22. However, conductance limitations of the plenum 34 can cause an uneven distribution of gas around the plenum 34 and consequently an uneven distribution of flow/pressure around the helical channels 26 of the downstream annular chamber 22. This will in turn cause poor stage performance and hence poor interstage inlet performance. Where the gas load arriving at the interstage inlet 32 far exceeds that from the first inlet and any other inlets located upstream from the Holweck mechanism, the negative behaviour of the poor distribution of the interstage gas load can dominate the performance of the Holweck mechanism.
In its preferred embodiments, the present invention seeks to improve the supply of gas to a pumping mechanism.